1. Field of the Invention
The present invention relates to a glass preform manufacturing method, which can accurately control the dopant concentration with low consumption of energy, and particularly, to a glass preform manufacturing method which is adapted to manufacture a low-loss optical fiber.
2. Background Art
Optical fibers produced using silica glass in which alkali metal oxide or alkaline earth metal oxide is doped has been shown by a number of predecessors to reduce the transmission loss thereof.
However, a technique for industrially mass-producing such optical fibers is incomplete.
In order to use a hydrolysis reaction or a thermal oxidation reaction caused by oxygen in gaseous phase in a conventional manufacturing method an optical fiber, it is necessary to use, for example, gaseous raw materials, such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4).
However, alkali metal ions or alkaline earth metal ions which are so-called hard cations form very strong ionic bonds.
For this reason, most of compounds (salts) formed by such bonds become solid at around ambient temperature and ambient pressure.
Accordingly, since the salts which become gas are hardly formed, it is difficult to apply the salts to the manufacture of optical fibers.
Therefore, in order to commercially produce optical fibers in which alkali metal oxide or alkaline earth metal oxide is doped, a manufacturing method different from the methods conventionally established in this field should be developed.
Various attempts have been taken in regard to such problems.
For example, a method of utilizing the characteristic that an alkali metal compound or an alkaline earth metal compound is easily soluble in water, mixing an aqueous solution of the alkali metal compound in the form of an aqueous mist in a source material gas to introduce the mist into an oxyhydrogen flame, and performing hydrolysis simultaneously with other raw materials to form glass has been attempted (for example, refer to Japanese Examined Patent Application, Second Publication No. S59-13453 and Japanese Examined Patent Application, Second Publication No. 559-14412).
Additionally, a technique of spraying the alkali metal compound onto an over-cladding as an aqueous solution simultaneously with a plasma-enhanced chemical vapor deposition method is disclosed in PCT International Publication No. WO 2009/034413.
Additionally, it is known that some composite salts obtained by reacting certain kinds of alkali metal salts and other metal salts with each other have vapor pressures higher than that of original alkali metal salts. The attempt to use these composite salts as raw materials is also performed (for example, refer to Japanese Patent No. 1787027).
Moreover, in recent years, a method of strongly heating an alkali metal halide or an alkaline earth metal halide to generate alkali metal vapor or alkaline earth metal vapor, exposing an optical fiber precursor glass to this alkali metal vapor, and doping alkali metal to the glass has been attempted (for example, refer to Published Japanese Translation No. 2007-516829 of PCT International Publication and Published Japanese Translation No. 2007.513862 of PCT International Publication, and PCT International Publication No. WO 2006/068941).
The above-described methods of using an aqueous solution of an alkali metal compound or an alkaline earth metal compound are objectionable manufacturing methods from the viewpoint that mixing of moisture which causes an increase in loss in manufacture of optical fibers should be ordinary avoided.
Additionally, in the above-described method of forming composite salts with higher vapor pressure and introducing the composite salts as vapor, the degree of rise in the vapor pressure is small, and the advantage is very restricted. Since some extra chemical species which are ordinary unnecessary for the function of an optical fiber are added in the manufacturing method, there is a concern that a rise in the transmission loss is brought about.
Moreover, the method of strongly heating an alkali metal compound or an alkaline earth metal compound and obtaining alkali metal vapor or alkaline earth metal vapor has an indefinite reaction mechanism of a reduction reaction, and is deficient in practice.